1. Field of the Invention
The present invention relates generally to hydraulic valves and more particularly hydraulic alternating valves which may be utilized with devices requiring selected control of two hydraulic cylinders. Still more specifically, the preferred embodiment of the present invention relates to an alternating valve which may be utilized with a tractor pulled planter having row marker arms which are alternately hydraulically lowered as the planter changes direction in the field.
2. Description of the Prior Art
It is known in the farm equipment art that planters may be equipped with row markers which are selectively movable from a raised to a lowered position depending upon the particular step in the planting process. As described in U.S. Pat. No. 4,285,268 issued Aug. 25, 1981 to Deckler for "Automatic Sequencing Valve And System", it is desirable that row markers be used with a tractor-drawn or mounted implement such as a multi-row planter. In such planters, the ground wheels are at least partially retracted during one pass so that the planter is lowered into a planting position. To make sure that the field is completely covered during the planting operation, one marker on one end of the planter is lowered, such marker typically including a disc to make a suitable mark in the ground. When the farmer reaches the end of one pass, the marker arm is raised and the tractor is turned around for the next pass. The driver will then follow the mark made by the marker disc so that the planter will be spaced a desirable distance from the rows planted in the preceding pass. During this second pass, it is typical for the other marker arm to be lowered. The process is then repeated until the entire field has been covered.
The valve disclosed in the Deckler patent is an automatic, hydraulically actuated sequencing valve which includes a valve body, a bore and a-valve spool which is axially movable in the bore. The move is caused by creating a differential hydraulic pressure on opposite ends of the valve spool during each admission of hydraulic fluid under actuating pressure to the valve.
As in the Deckler system, most systems known to the art have been tied to the hydraulic lift cylinder on the planter itself, so that when the planter is raised the marker arms are both raised to the full up position, and when the planter is lowered, the proper marker arm is lowered. The Deckler system depends on pressure drops through small orifices to shift the spool. Pressure drop systems are known to be dependent on oil viscosity, flow rates and the loads imposed on the system. It has been found that this type of system is susceptible to malfunctions as conditions change, e.g. a change in the oil viscosity or flow rate.
Another type of alternating valve for such uses is shown in U.S. Pat. No. 4,488,476 issued Dec. 18, 1984 to Diel, et al for "Marker Control Valve". The valve of this patent includes a cam operated alternator valve spool that rotates the alternator spool 180.degree. when the planter is lowered. It is believed by the present inventor that this method of alternating overcomes many of the problems of the Deckler type system but still has a major drawback. That major drawback is the expense required to manufacture the cam means used in the alternator valve spool. As shown in the Figures of that patent, the cam device includes an elaborate groove machined into the spool which rotates the spool 180.degree. each time the planter is lowered. Such machining is labor intensive, slow and hence expensive.
It would represent a significant advance in this art if a valve could be developed which did not suffer from either disadvantage of the aforementioned prior art devices, especially if reliable operational results could be achieved using a valve spool which could be cast without requiring expensive machining operations.